1. Field of the Invention
The present invention relates to a hydraulic control device for an automatic transmission of an engine, and in particular to a hydraulic control device for an automatic transmission mounted on an automobile. Even more specifically, the present invention relates to an engine-brake control device for controlling hydraulic pressure in a brake hydraulic servo of a brake directly restraining a friction-engaging element of a shift gear mechanism.
2. Description of the Prior Art
Conventionally, an automatic transmission effects an engine-brake in a second range (or S range) or in an L range by actuating a second-coast brake (at 2nd speed) to retrain a front and rear sun gear, or by actuating a first & reverse brake (at 1st speed).
Generally, hydraulic servos for the second-coast brake and the first & reverse brake are supplied with modulated pressure from a second-coast modulator valve or a low-coast modulator valve.
As shown in Japanese Laid Open Patent No. Sho-62-77245, with reference to an exhaust brake attached automatic transmission, an engine-brake control device has been proposed in which line pressure is led to a low-coast modulator valve through a governor valve to vary a hydraulic pressure for the engine-brake according to the vehicle speed.
The above-mentioned modulator valve reduces the line pressure and supplies it to the hydraulic servos of the second-coast brake and the first & reverse brake so that shift shock is reduced. However, in the modulator valve, as represented in FIG. 6 by dotted lines, the pressure is constantly reduced regardless of vehicle speed.
Consequently, even if a manual valve is shifted to the second range or the L range at a high speed, the engine-brake is not sufficiently carried out because the second-coast brake or the first & reverse brake are not provided with enough torque. On the contrary, when a vehicle is running at low speed, the brake torque is too large compared to vehicle inertia during low speed running, so that the engine-brake is effected with too strong of a force. Due to these occurrences, the shifting operation is quite laborious, and riding comfort is poor because the engine-brake is delayed at high speeds and is effected by a strong force at low speeds.
With reference to the above-mentioned device which varies the hydraulic pressure in the brake servo according to vehicle speed by the governor valve, as shown in FIG. 10, a brake torque capacity X based on the governor valve is designed to be small within a middle speed area Q because of a centrifugal force generated by the governor weight, while the torque capacity is designed to be large within high speed area R. Subsequently, the brake torque capacity necessary for facilitating an engine-brake Y is not enough, so that slip occurs between the brake and a gear element. In the high speed area R, the brake torque capacity is too large compared to that necessary to effect the engine-brake Y, so that a sharp shift shock occurs.
When a foot brake is applied during the engine-brake, in particular, if the hydraulic pressure of engine-brake hydraulic servo is designed to vary with the vehicle speed, the driving wheels may lock. Furthermore, when a vehicle runs on roads having a low coefficient of friction such as those covered with snow, the driving wheels may slip because the torque required for effecting an engine-brake is too large.